Conventional low density polyethylene (LDPE) has good processability; however, when used in film and/or extrusion coating applications, increased melt strength is still desired.
U.S. Publication No. 2008/0242809 discloses a process for preparing an ethylene copolymer, where the polymerization takes place in a tubular reactor at a peak temperature between 290° C. and 350° C. The comonomer is a di- or higher functional (meth)acrylate used in an amount of 0.008-0.200 mole percent relative to the amount of ethylene copolymer.
International Publication No. WO 2007/110127 discloses an extrusion coating composition comprising an ethylene copolymer obtained by polymerization in a tubular reactor at a peak temperature of 300-350° C. The comonomer is a bifunctional α, ω-alkadiene.
International Publication No. WO 97/45465 discloses an unsaturated ethylene copolymer, a method for producing it, and its use for producing crosslinked structures. The unsaturated ethylene copolymer comprises a polymer, obtained by radical polymerization through a high-pressure process, of ethylene and at least one monomer which is copolymerizable with ethylene and includes a diunsaturated comonomer of the formula (I): H2C═CH—O—R—CH═CH2, wherein R=—(CH2)m-O—, —(CH2CH2O)n-, or —CH2-C6H10-CH2-O—, m=2-10, and n=1-5. Preferably, the comonomer of formula (I) is 1,4-butanediol divinyl ether.
International Publication No. WO 2012/057975 describes the need for polymers comprising monomeric chain transfer agents (CTAs), and showing improved melt strength and a low soluble content. International Publication No. WO 2012/084787 describes simulated tubular reactions in which bi- and/or higher functional comonomers are used to increase the long chain branching (LCB) level in the polymer. These bi- and/or higher functional comonomers have at least one vinyl group and at least one CTA group, by which LCB or T-branches can be formed, after initial incorporation into the ethylene-based Polymer, by the first functionality, and further reaction of the remaining functionality.
International Application No. PCT/US13/029881 (now WO 2014/003837) discloses an ethylene-based polymer, formed from reacting ethylene and at least one asymmetrical polyene comprising an “alpha, beta unsaturated end” and a “C—C double bond end,” wherein the reaction takes place in the presence of at least one free-radical initiator.
The impact of the above-described multifunctional components on the final polymer through coupling and/or branching reactions is complex, and depends on the type and reactivity of the functional groups. A vinyl functional group will act as a comonomer and incorporate into a polymer chain/molecule. When involved, CTA functionality will either start the formation of a new polymer molecule, or initiate, after incorporation of the monomeric group, the formation of a LCB or T-branch. For a multi- and/or bifunctional component to impact polymer rheology, it is important that (1) at least two functional groups of the component molecule react and (2) effective branches are formed in the polymer.
H-branches are either intermolecular (between two molecules) or intra-molecular (within a molecule) and formed by reaction of two or more vinyl groups of the bi- and/or multifunctional component. The probability that functional groups will react, and contribute to a melt strength increase, depends on the reactivity of the functional groups, overall, and remaining conversion level, and molecular topology of the polymer, showing how the component is incorporated by its first reacting functionality. The impact of H-branch formation on melt strength will be (1) negligible with intra-molecular H-branch formation, (2) low for intermolecular H-branch formation between two small polymer molecules, and (3) significant for intermolecular H-branch formation between two larger molecules. However, the latter (3) could lead to the formation of gels, especially when crosslinked networks are formed between and inside large polymer molecules.
Taking into account the reaction kinetic data reported by Ehrlich and Mortimer in Adv. Polymer Sci., Vol 7, pp. 386-448 (1970), and a typical ethylene conversion level in a tubular reactor of 25-35%, the following general remarks can be made: (i) the incorporation level per reactor pass is less than 50% for hydrocarbon dienes, while the probability of forming H-branches is less than 10%; (ii) a monomeric CTA containing acrylate monomer functionality will have a high incorporation level per reactor pass, but further reaction would be required to form a T-branch; and (iii) the probability that the CTA functionality will react, depends on chain transfer activity and remaining conversion level. For compounds with a CTA functionality similar to typically used CTAs for the high pressure LDPE process, the amount of T-branching formed would be low. Di- or higher functional (meth)acrylate components will lead to almost complete incorporation in the polymer, and a very high level of secondary reaction. The high reactivity of the functional groups makes even distribution over the polymer formed in a tubular reactor difficult. Furthermore, coupling or H-branch formation, when the component is fed to the first reaction zone, will already occur in the first reaction zone, thus increasing the risk of initiating and/or forming product gels and fouling in the first reaction zone, with further exposure and deterioration in the remaining reaction/cooling zones if present.
International Publication No. WO2013/059042 describes the use of fresh ethylene and/or CTA feed distribution to broaden molecular weight distribution (MWD) and increase melt strength, while remaining all other process conditions constant.
Liu, J., et al., Branched Polymer via Free Radical Polymerization of Chain Transfer Monomer: A Theoretical and Experimental Investigation, J. Polym. Sci. Part A: Polym. Chem., (2007), 46, 1449-59, discloses a mathematical model for the free radical polymerization of chain transfer monomers containing both polymerizable vinyl groups and telogen groups. The molecular architecture of the polymer is disclosed as being prognosticated, according to the developed model, which was validated experimentally by the homopolymerization of 4-vinyl benzyl thiol (VBT), and its copolymerization with styrene.
Wu, P-C et al, Monte Carlo simulation of structure of Low-Density Polyethylene; Ind. Eng. Chem. Prod. Res. Develop., Vol. 11, No 3, 352-357 (1972) describes modeling polymer molecule formation through consecutive branching by applying a Monte Carlo simulation.
Iedema, P. D. et al., Rheological Characterization of Computationally Synthesized Reactor Populations of Hyperbranched Macromolecules; Bivarate Seniority-Priority Distribution of ldPE, Macromolcular Theory and Simulations, 13, 400-418 (2004) shows computationally obtained seniority and priority distributions, in order to characterize LDPE, using rheological quantities, in terms of comb-shaped and Cayley tree structures.
The continuous stirred tank reactor (CSTR), or autoclave, process typically leads to more Cayley tree structured molecular polymer topology, due to the inherent, more homogeneous LCB level and chain segment size distribution, while the residence time distribution creates very long and very short growth paths, leading to a broader MWD. Furthermore, a comonomer will be homogenously incorporated within a reaction zone regardless of its reactivity. The tubular reactor process typically leads to more comb-shaped molecular polymer topology, due to the low starting LCB level and the lower temperature conditions leading to long chain segments, while the MWD is narrowed, due to the more homogenous residence time distribution. However, the lack of back mixing, as present in a CSTR reactor, or axial mixing, leads to a comonomer incorporation distribution that is strongly affected by the reactivity of the comonomer and the changing composition of reactants along the tubular reactor. Comonomers with reactivity similar to ethylene, such as vinyl acetate, will lead to homogeneous copolymers, while highly reactive comonomers, such as acrylates, will lead to strongly inhomogeneous copolymers in a tubular reactor. Incorporation in a larger polymer molecule, at a position more inside the polymer molecule sphere (higher priority and seniority in Cayley tree structure), may affect reactivity and increase the probability for an intra-molecular reaction. Incorporation in smaller (lower gyration radius), and/or linear polymer molecules (comb like structure), and/or at a position more at the outer side of polymer molecule sphere (lower priority and seniority in a Cayley tree structure), may affect the reactivity less, and increase the probability for an intermolecular reaction.
The use of highly reactive multifunctional components is also subject to other process and/or polymer concerns, such as stability of the component, premature polymerization and fouling formation potential in the compression and preheating sections, reactor fouling, gel formation in polymer, process stability potentially leading to ethylene decomposition, and the like.
International Publication No. WO 2013/149699 describes improving the purity and/or stability of non-conjugated double bonds to reduce the percentage conversion in the so-called “zero conversion test.” The “zero conversion test” simulates the polymerization and fouling potential of a non-conjugated diene in a tubular reactor preheating section, prior to starting the reaction through injection/activation of an initiator.
International Publication No. WO2013/149698 describes using an inhibitor to prevent unwanted polymerization (i.e., premature polymerization) at the preheater walls, prior to addition of the free radical initiator, when applying a non-conjugated diene. International Publication No. WO2013/132011 describes preventing preheating fouling, by feeding the non-conjugated diene after preheating the ethylene, and before starting the reaction through injection/activation with an initiator.
There remains a need for such processes that can be used in combination with coupling and/or branching components with differentiated monomeric reactivity, to form ethylene-based polymers, such as LDPE, with improved melt strength, especially for film and extrusion coating applications. There is a further need for such processes that make optimal use of the coupling and/or branching component, while avoiding and minimizing preheater fouling and gel formation. These needs and others have been met by the following invention.